Added post about removing a device feature

content/posts/remove-device-feature.md

@@ -0,0 +1,131 @@
+---
+title: "How to remove a Device Feature"
+date: 2023-08-07T13:12:43+02:00
+draft: false
+author: "Florian Bramer"
+---
+
+Recently, I had to remove a couple of device features from an Android 12 build.
+
+Some reasons why we wanted to remove them:
+
+- they were actually not supported by our device
+- we didn't want to expose them to the user
+- they were causing unnecessary CTS failures
+
+In this post, I want to guide you through the steps I took to trim down the list of declared device features.
+
+### What are Device Features
+
+Any android devices declares a list of device features it supports. For example, in the early years of Android, there where phones with and without a built in compass sensor.
+
+Imagine you are an app developer working on a compass app and you want to assure that your app only gets installed on phones with compass sensor.
+
+You could archive that by declaring a dependency onto the corresponding device feature in your `AndroidManifest.xml`:
+
+```
+<manifest ... >
+    <uses-feature android:name="android.hardware.sensor.compass"
+                  android:required="true" />
+    ...
+</manifest>
+```
+
+See https://developer.android.com/guide/topics/manifest/uses-feature-element#features-reference for a list of device features.
+
+### Inspect all exposed Device Features
+
+First let's inspect all the device features our device declares. One can do that by using an `adb shell` session and Android's package manager.
+
+```
+> pm list features
+feature:android.hardware.audio.output
+feature:android.hardware.bluetooth
+feature:android.hardware.bluetooth_le
+feature:android.hardware.broadcastradio
+feature:android.hardware.camera.any
+feature:android.hardware.camera.autofocus
+feature:android.hardware.ethernet
+feature:android.hardware.faketouch
+feature:android.hardware.location
+feature:android.hardware.location.gps
+feature:android.hardware.location.network
+...
+<many more>
+...
+```
+
+Let's take a deeper look how this list is assembled.
+
+The package manager calculates the list from the .xml files it can find in the these folders
+
+- vendor/etc/permissions
+- system/etc/permissions
+- system_ext/etc/permissions
+
+during startup. For example at
+
+_vendor/etc/permissions/android.hardware.bluetooth_le.xml_
+
+we can find the file which declares the `android.hardware.bluetooth_le` device feature we saw in the list above:
+
+```
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2013 The Android Open Source Project
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+<!-- Adds the feature indicating support for the Bluetooth Low Energy API -->
+<permissions>
+    <feature name="android.hardware.bluetooth_le" />
+</permissions>
+```
+
+### Remove individual Device Features
+
+Now let's imagine we would like to remove `android.hardware.bluetooth_le` from the list of declared device features. An obvious way to do that, would be to find the _android.hardware.bluetooth_le.xml_ file in our source tree and remove (or comment out) the line which defines the feature:
+
+_frameworks/native/data/etc/android.hardware.bluetooth_le.xml_:
+```
+<permissions>
+<!--
+    Purposly commented out because we don't want to support BT Low Energy on our device
+    <feature name="android.hardware.bluetooth_le" />
+-->
+</permissions>
+```
+
+But this solution comes with a couple of drawbacks:
+- By AOSP convention, individual device configurations should go into the _device/_ folder. Your co-workers might not find this change if you place it underneath _frameworks/native_
+- Once a new Android version gets introduced into the project, your change might get overwritten
+
+A more sustainable way to trim the list of device features would be to introduce a new .xml file in the device folder of your device.
+
+e.g. _unavailable-features.xml_:
+
+```
+<?xml version="1.0" encoding="utf-8"?>
+
+<permissions>
+    <!-- we don't want to support BT Low Energy on our device -->
+    <unavailable-feature name="android.hardware.bluetooth_le" />
+</permissions>
+```
+
+All you need to do then is to copy the .xml during built time onto the vendor partition in one of your .mk files:
+
+```
+PRODUCT_COPY_FILES += device/<vendor>/<device>/unavailable-features.xml:vendor/etc/permissions/unavailable-features.xml
+```
+
+Afterwards the package manager will include your .xml into his calculations and `android.hardware.bluetooth_le` will disappear from the list of declared device features.